[geeks] fwd: The Seven Phases of Owning an iPod - An Illustrated Journey

[John Francini]

On 28 Nov 2006, at 16:11, velociraptor wrote:

> On 11/23/06, Lionel Peterson <lionel4287 at verizon.net> wrote:
>>> From: John Francini <francini at mac.com>
>>> Date: 2006/11/21 Tue AM 10:55:27 CST
>>> To: The Geeks List <geeks at sunhelp.org>
>>> Subject: Re: [geeks] fwd: The Seven Phases of Owning an iPod - An  
>>> Illustrated Journey
>>
>>> Well, the FCC-mandated limits on things like frequency response,
>>> power, and modulation pretty much
>>> guarantee that the FM signal won't be as good as a cassette adapter.
>>
>> What?
>>
>> The little transmitters (in the US of A) are almost always covered  
>> by Part 15, which limits radiated power and antenna length,  
>> nothing more. As I understand it any modulation is aceptable,  
>> without limitations, but the user of a Part 15 device is not  
>> allowed to disrupt other, licensed transmissions (like a cell  
>> phone jammer, etc.).
>
> This is the regulation that the FCC used put the stops on XM & Sirius
> for--reports that their transmitters were interfering with other cars'
> FM reception.  I think they both got through that recently though.

The monaural audio bandpass of a standard FM stereo signal  
(equivalent of L + R) runs from 30 Hz to 15 kHz.

In order to send a stereo signal, the L-R difference signal is  
imposed upon a carrier-suppressed double sideband signal centered at  
38 kHz.
This is detected in the receiver by the presence of a 19 kHz 'stereo  
pilot tone'.  A 4kHz guard band is required around this tone; hence  
the limitation of the audio signal to 15 kHz.

Also, it's common practice to limit the dynamic range of the signal  
to stay within the bandwidth limits.

What it all adds up to is simple: neither the FM audio bandpass nor  
the dynamic range  aren't as wide as what can be stored on a CD, so  
the signal from the iPod to the car stereo will suffer in comparison  
with a direct digital hookup, a direct analog hookup, or even a  
cassette adapter.

More info on FM stereo modulation: <http://en.wikipedia.org/wiki/ 
FM_stereo>